Carburetor having a heat pipe for vaporizing fuel

ABSTRACT

A fuel vaporizing system for spark ignition internal combustion engines wherein a heat pipe filled with a narrow-range boiling point fluid is disposed to transfer heat from a heat zone to a vaporizing zone. The liquid fuel is vaporized in the vaporizing zone prior to being mixed with air.

United States Patent 1 1 1 1 93,%38

Lindsay et a1. 01st. 9, 1973 CARBURETOR HAVING A HEAT PIPE FOR [56]References Cited VAPORIZING FUEL UNITED STATES PATENTS lnvemorsl RogerLind'say, Tattenhall; Alun 3,273,983 9/1966 Minoza 123/133 T 1 r both fEng 3,229,759 1/1966 Grover.... 165/105 Ian C. Finlay, Glasgow; John L.2,240,311 4/1941 Mills 123/122 AA wi|s0n,EaStKi1bridge,bO[h f 2,472,7176/1949 Morey 165/105 Scotland 2,766,974 10/1956 McConnell 165/1053,150,651 9/1964 Buchner et al 1 123/122 E 7 A 2 3,332,476 7/1967McDougal 165/105 3] Sslgnee Company New 3,253,647 5/1966 Deshaies123/122 E [22] Filed: Dec. 23, 1971 [2!] App} 2 52 Primary Examiner-C.J. Husar Attorney-Theodore E. Bieber et a].

[] Foreign Application Priority Data Dec. 23, 1970 Great Britain 61,041/[57] ABSTRACT A fuel vaporizing system for spark ignition internal 1 Cl3/ 123/12 123/133, combustion engines wherein a heat pipe filled with a/32, 165/105 narrow-range boiling point fluid is disposed to transfer[51] Int. Cl. F02!!! 13/06 heat from a, heat one to a vaporizing zone,The liquid Field of Search l23/l22 122 fuel is vaporized in thevaporizing zone prior to being 1 165/105, 3 mixed with air.

V 14 Claims, 13 Drawing Figures CARBURETOR HAVING A HEAT PIPE FORVAPORIZING FUEL BACKGROUND OF THE INVENTION This invention relates to afuel vaporizing device for use with a spark-ignition internal combustionengine.

The conventional spark-ignition relies on a carburetor to mix a desiredproportion of a volatile fuel with the inlet air. For completecombustion of the fuel-air mixture the fuel needs to be uniformlydispersed in the air. Such a uniform mixture is seldom if ever obtainedin practice with a conventional carburetor. Manifold fuel injectionoffers only marginal improvement over the results obtained withcarburetors.

SUMMARY OF THE INVENTION The invention seeks to enable a more uniformfuelair mixture to be obtained.

This is achieved in accordance with the invention by completelyvaporizing the fuel by boiling an intermediate heat transfer liquidhaving a narrow range boiling point and enclosed in a sealed containerand using the latent heat of condensation ofits vapor to vaporizesubstantially the entire fuel charge prior to its admixture with thecombustion air. Evaporation of the fuel thus takes place at asubstantially constant temperature irrespective of the rate at which itis demanded.

When the fuel vapor is mixed with the main stream of combustion airwhich is relatively cool the vapor may condense into very smalldroplets, in the form of an aerosol, which not only tend to dispersevery rapidly to produce a uniform fuel-air mixture, but also resist anytendency to accumulate liquid on surfaces that it may encounter, e.g.,on the walls of the inlet manifold.

During normal operation, the heat to boil the liquid is convenientlydrawn from the exhaust gases.

A fuel-vaporizing device in accordance with the invention includes oneor more heat pipes (as specified below) each of which is in the form ofa sealed vessel containing a heat transfer fluid so constructed that inoperation the liquid phase of the fluid is arranged to collect in a heatreceiving zone and the vapour phase or the fluid is arranged to condensein a heat discharging zone, so that the latent heat of vaporization ofthe heat transfer fluid is used to vaporize liquid fuel upstream of thepoint at which it is to be mixed with the main stream of combustion air.

The term heat pipe as used in this specification is intended to includedevices known as two-phase thermosyphons. This latter term is derivedfrom the presence of both liquid and vapour phases in the device. Thegeometry of the heat pipe is is no way limited to a circular or tubularconfiguration.

The use of one or more heat pipes enables heat to be supplied to thefuel within a restricted temperature range virtually regardless of therate at which the fuel is demanded. Furthermore, at startup the heatpipe or pipes reach their operating temperature very much more quicklythan a solid heat conductor.

The vaporizing device may be used in conjunction with a suitablymodified conventional carburetor, or, better still, with a fuelinjection system.

The heat-receiving zone of each heat pipe is conveniently arranged toreceive heat from the exhaust gases. An auxiliary heat source may beprovided for use during cold startups.

As the heat demanded from the heat pipe may vary considerably, surplusheat may be removed by conventional cooling means.

The inclusion of a non-condensable gas in each heat pipe can be arrangedto act as a buffer in order to regulate the cooling of the heat transferfluid.

In operation, the non-condensable gas will be driven to a relativelycool part of the vessel by the vapor pressure of the heat transfer fluidand progressively compressed as the heat contained in the heat pipeincreases in excess of requirements. The properties of the vapor risingin the tube past the level of the cooling zone will determine the lossof surplus heat and thus prevent overheating.

The invention will now be further described by way of example withreference to the accompanying drawings in which:

FIG. 1 is a diagrammatic sectional side elevation of a heat pipe;

FIG. 2 is a diagrammatic sectional side elevation of a heat pipecontaining a wick;

FIG. 3 is similar to FIG. I in all respects except that it additionallycontains a quantity of non-condensable gas;

FIG. 4 is similar to FIG. 3, but differs in its operating condition;

FIG. 5 is a diagrammatic sectional side elevation of a vaporizing devicein accordance with the invention;

FIG. 6 is similar to FIG. 5 except that the heat pipe additionallycontains a quantity of non-condensable FIG. 7 is a modified form of theinvention;

FIG. 8 is a vertical section of a practical embodiment of the invention;

FIG. 9 is a horizontal section of the embodiment shown in FIG. 7;

FIG. 10 is a vertical section of a modified form of the invention shownin FIGS. 7 and 8;

FIG. 11 is a vertical section of a second modified form of the inventionshown in FIGS. 7 and 8;

FIG. 12 is a vertical section of a second practical embodiment of theinvention; and

FIG. 13 is a horizontal section of the embodiment shown in FIG. II.

The heat pipe in FIG. 1 comprises a sealed vessel 10 containing a smallquantity of a heat transfer liquid 12 which has a narrow-range boilingpoint. The space above the surface 14 of the liquid 12 is occupied byits vapor 16.

In operation, the heat pipe receives heat in the zone marked A, whichcauses the liquid 12 to boil. The resulting vapor passes up the pipe andcondenses on cooler surfaces, thereby heating them virtually to theboiling point of the liquid. Heat is thus available at a heat transferzone B at a substantially constant temperature. When heat is removed, itcauses a proportion of the vapor 16 to condense and to collect in theheat receiving zone A. When more heat is being supplied at A than isrequired at B, the vapor rises further up the container 10 into acooling zone C where the vapor is condensed by external cooling means.

The heat pipe in FIG. I relies on gravity to return the condensate tothe heat receiving zone A, which restricts its use to situations inwhich this will occur.

An embodiment of a heat pipe which does not rely on gravity to returnthe liquid to the zone A is shown in FIG. 2. The heat pipe which in allother respects may be similar to that shown in FIG. 1 or 3 is providedwith a wick-like lining 17 on its inner walls.

In operation the liquid 12 is evaporated in zone A by the heat suppliedto the heat pipe, and the capillary of the liquid in the wick drawsfresh liquid into the evaporating zone A. The vapor 16 is caused tocirculate in the direction indicated by the cooling in the zone B, whichcauses contraction in the volume of the vapor and condensation into thewick.

In the embodiment of the heat pipe shown in FIG. 3, a small quantity ofnon-condensable gas 18 is introduced into the space above the liquidsurface 14. This gas is progressively forced to the cooler zone C of theheat pipe as the temperature of the heat pipe increases. This is causedby the flux of vapor from the lower part of the container to the coolerupper part in which the vapor tends to condense. The result is thatthere is effectively an interface at 20 between the vapor l6 and the gas18, caused by the temperature variation along the heat pipe. If the fluxof vapor exceeds that needed to satisfy the thermal demands of zone B,it tends to cause the interface 20 to lift to expose a greater part ofthe cooling zone C to the vapor 16 (as shown in FIG. 4). In this way therate of removal of surplus heat from the heat pipe is effectivelyregulated. In either embodiment, should a sudden increase in heat demandoccur, the more rapid condensation of the vapor 16 causes a drop intotal pressure inside the vessel 10 so that the liquid boils morequickly to restore equilibrium.

In the embodiment of FIG. 3 the interface 20 will similarly move downthe tube.

Although the heat pipes shown are tubular, their exact geometry will bedetermined by consideration of their required thermal characteristicsand their working environment.

The fuel vaporizing device shown in FIG. comprises a heat pipe whoseheat receivng zone A is arranged to receive heat from exhaust gases inan exhaust passage 22.

A fuel line 24 is disposed in the heat transfer zone B of the heat pipeto receive sufficient heat, at constant temperature, to insure that allthe fuel is vaporized prior to its admixture with the main stream ofcombustion air in the inlet manifold 26. On mixing with the relativelycold main air stream the vaporized fuel may condense to form a mist oraerosol of fine droplets whose diameter may be as small as 1.0 [.L.

The necessary fuel metering equipment is not shown, but it must becapable of delivering an appropriate quantity of fuel depending on suchfactors as engine speed and load and the position of the butterfly valve28 in the inlet manifold.

FIG. 6 is similar to FIG. 5, save that the heat pipe 10 has beenstabilized by the provision of cooling means 30 at its upper end. Thevapor 16 of the heat transfer fluid is normally kept out of contact withthis part of the heat pipe by a small quantity of non-condensable gas 32which behaves in the way described with reference to FIG. 3.

FIG. 7 differs from FIG. 6 only in that the fuel is vaporized in theheat transfer region B in the presence of a small quantity of air bledfrom the main stream of combustion air through a by-pass passage 34. Thefuel vapor and air mixture are returned to the main stream of combustionair via a passage 36, The advantage of this construction is that thefuel vapor-air mixture may mix with the main stream more quickly. Theproportion of air bled through the passages 34, 36 should not be so highthat any significant deterioration of volumetric efficienty resultsthrough the increase in inlet air temperature.

A practical embodiment of a vaporizing device is shown in FIGS. 8 and 9.The right-hand end 40 of the device as shown is intended for connectionto the engine inlet port so that the flow of air is in a left-to-rightdirection. On entering the left-hand end 42 of the device it passesthrough a conventional, though modified, carburetor 44. The air streamis then divided into a primary and secondardy stream, the lattercontaining an over-rich mixture of fuel droplets with air, while theformer comprises only air. After vaporization of the fuel in thesecondary stream, the two streams are reconbined.

The primary and secondary streams are segregated by a pair oflongitudinal partitions 46 extending from upstream (i.e., to the left)of the point at which fuel enters through the carburetor to a remixingpoint downstream of the heat transfer zone 47 of the vaporizing device.The primary stream passes along the outer duct 48 which consists of aventuri which is formed by contraction and expansion of thecorss-sectional area, while the secondary stream passes along thecentral duct 50.

The operation of the carburetor 44 is well known, and forms no part ofthe invention as it serves simply as one of many ways of supplying ametered fuel charge to the engine inlet air. The most notablemodification is that the butterfly valve 52 is placed downstream of thevaporizing device. Otherwise the carburetor operates normally; that isto say, the piston 54 is lifted by the vacuum downstream of thecarburetor, causing both the throttle valve 56 to lift, andsimultaneously tapered needle 58 which regulates the area of the fuelorifice 60, Fuel is drawn from the orifice 60 by the vacuum. Control ofthe response of the piston 54 is effectived by a damper 62, a spring 64and the size of the orifice 66 communicating the upper side of thepiston with the downstream side of the valve 56. In order to accommodatethe partitions 46, slots 68 are milled in the throttle valve 56.

The heat transfer zone 47 of the device comprises a number of uprighttubular sections 70 extending through the central duct 50. Thesesections 70 correspond to the zone marked B in the previous Figures. Theheat-receiving portion of the heat pipe 10 projects into the engineexhaust manifold 22 and is filled with liquid 12. Water-cooling means 51provided at the upper end of the heat pipe. During normal operation ofthe device, the extent to which the vapor 16 comes into contact with thecooling means 51 is determined by means of a non-condensing gas at 32 asalready described.

The carburetor can be replaced by a suitable fuel injection systemwithout the need for any major change in the configuration of thevaporizing device.

In order to keep the quantity of fuel residing at any one instant oftime on the evaporator surfaces during the process of evaporation, it isnecessary that a small surface area is used for evaporation, and thisshould not exceed 200 cm for each gram of hydrocarbon fuel to beevaporated per second. The air velocity across the evaporator surfacesat full throttle should be maintained at a velocity of at least 25 ms",thereby improving the rate of evaporation of the fuel. The process isassisted by introducing the liquid fuel into the evaporating device inthe form of droplets, and the unevaporated liquid passes from surface tosurface as a liquid spray carried in the air stream. This mist flowsubstantially improves the heat transfer rate from the metal surface tothe liquid film.

As a consequence of the required air velocity, the pressure drop in thesecondary duct cannot be neglected. Only a proportion of the total airflow passes over the heated tubes in the secondary duct, the remainderflowing through the primary ducts. The primary ducts are formed in thefollowing manner to reduce the pressure loss through the evaporatingdevice. The primary ducts are in the form of a venturi. The convergentpart increases the air velocity and thereby decreases its staticpressure. At the minimum cross section and, therefore, the minimumpressure, the secondary duct is recombined with the primary ducts. Thislow pressure induces the flow through the secondary duct. The recombinedduct is made divergent reducing the gas velocity and thus raising thepressure. The overall pressure drop acrossthe evaporating device is,thereofore, less than the pressure drop across the evaporating tubesalone. The position of the partitions 46 dividing the inlet flow intothe primary and secondary streams is such that the relative areas of theducts are in the proportions selected for the primary and secondaryflows. The minimum area of the primary ducts in arranged to provide thepressure drop calculated to be required by the secondary air flowingacross the evaporator tubes. This ensures an equal velocity of each airstream at the separation of the flows, giving a negligible distrubanceto the flow.

It is preferable but not essential that the fuel is metered into thesecondary duct at a flow rate that compensates for the changing quantityof liquid fuel residing on the evaporating surface, thus providing aconstant air/fuel ratio at all times. This can be achieved by one ofseveral methods. I v

The first method is substantially a modification of a piston-controlledcarburetor wherein the needle valve that meters the fuel is carried by apiston whose position is controlled by the air velocity. A delay isprovided in the motion of this needle valve such that a temporaryincrease or decrease in the fuel supply compensates for the changingfuel on the evaporating surfaces. This delay may be obtained by theinertia of the mass of the piston or damping mechanism.

In the second method a transducer measures the air flow rate andproduces an electrical signal. An electrical circuit modifies thissignal in such a manner that when it is used to actuate a fuel-meteringvalve, a fuel flow is produced which compensates for the fuel residingon the evaporating surfaces, for the temperatures of various parts ofthe engine and the liquid fuel, and also for the air temperature andpressure.

In a third method, the quantity of air, as well as the quantity of fuel,is to be metered to the engine in such a manner that the air-flowremains constant, and in a quantity so called for by the position of theaccelerator pedal.

The device may also be modified as shown in FIG. 10, in which itincorporates a liquid return pipe 72 extending below the level 14 of theliquid in the heatreceiving position. The lower part 74 of the heat pipethen extends above the floor 76 of the central part 76 of the heat pipein order to provide a head which will insure that the condensate willdrain to the heatreceiving zone without it being interfered with by therising vapor in the heat pipe. A fuel injection nozzle 78 is shown inplace of the carburetor in FIGS. 8 and 9.

Another method of improving the circulation of the vapor and thecondensate so that there is no counterflow present is to introduce thevapor into the upper part of the central part 76 of the heat pipe. Thisis shown in FIG. 11. In this case the cooling means 51 is dispensedwith.

FIGS. 12 and 13 show a form in which the mechani cal construction of thedevice has beensimplified, and several discrete heat pipes 80 areemployed. This construction has the advantage that it will still operatein the event of a failure 'of one of the heat pipes. It may also bepossible to modify existing automobiles by employing a simplified systemof this kind.

Under operating conditions the liquid phase in the heat pipe should notoccupy more than 40 percent of the total internal volume of the heatpipe, the remainder of the internal volume being occupied by the vaporphase and the non-condensable gas when used. In liquid form the heatpipe fluid may be a pure substance or mixture such that the freezingtemperature lies between 0C and l00C and the boiling temperature atatmospheric pressure between C and 300C. It is to be chemically stableand non-corrosive to the materials of construction used at the operationtemperature of the thermosyphon or heat pipe and appreciable chemicaldecomposition or reaction should not occur within a period of severalyears when within the thermosyphon or heat pipe. Examples of a suitablefluid are 2-octanol, decane and tetralin.

In order to obtain a high heat flux in the heat pipe, it is advantageousfor the vapor of the heat transfer fluid to be as dense as possible.This tends to imply a high operating pressure in the heat pipe, but inorder to avoid a very heavy construction this should be kept to between1 and 4 atmospheres under normal conditions.

The non-condensable gas, if present,.should be gaseous at the operatingpressure and chemically nonreactive within the thermosyphon or heatpipe. Examples are nitrogen, helium, argon, neon and krypton.

In place of the stabilizing method proposed for the heat pipe, namely,using a non-condensable gas, it will be appreciated that instead ofregulating the heat loss from the heat pipe, the heat supplied to theheat pipe can be controlled by mechanical or electrical means, operatingin response to, say, the pressure in the heat pipe. An electrical methodis naturally to be preferred in the case where electrical energy is usedto heat the heat pipe.

' The choice of a suitable heat transfer fluid is important; it willdepend upon the characteristics of the fuel to be employed and, inparticular, the final boiling point (FBP) of the latter. The boilingpoint of the heat transfer fluid should be higher than the FBP of thefuel, but not so high that it causes deterioration of the fuel, such asby cracking, to occur. For example, for a typical gasoline whose F8? isC, the operating temperature of the heat pipe should not be lower than200C, nor higher than 300C. The quantity of noncondensable gas withinthe heat pipe will have a considerable effect on the stable operatingtemperature of the heat pipe and should thus be carefully controlled inmanufacture.

The construction of the heat pipe must also take into account themaximum and minimum fuel vaporizing requirements, and have sufficientcooling so that at no time is all of the heat transfer liquid 12 allowedto vaporize. Alternatively, however, complete evaporation of the liquid12 can be utilized to limit the maximum heat flux, which can beconveyed.

The use of the vaporizing device in accordance with the inventionenables a gasoline engine to be run on such lead mixtures, in excess of20:1 air-fuel ratio, that the levels of carbon monoxide and oxides ofnitrogen are simultaneously very low.

The capability of burning fuel at ultra lean mixtures also permits fuelhaving a lower octane number to be used with the same compression ratio.This is particularly important in that at the present time refiningtechniques in use cannot produce a lead-free fuel having the same highoctane ratings as the leaded premium grades.

In the gas turbine, the vaporizing device is used for vaporizing fuelprior to its mixture with air in the primary combustion zone. It is alsoused to inject vaporized fuel into oxygen-rich gases, in an inter-stageturbine reheat system in a power-reducing engine, or in an exhaustreheat system in a jet propulsion or vertical lift jet engine.

It is contemplated that it may be desirable to provide electric or otherheating means 18 in FIG. 1 for startup conditions before the exhaustgases can provide the necessary heat. This starting-up heat could besupplied to the fuel via an auxiliary heat pipe or pipes.

The use of the vaporizing device in accordance with the inventionenables a gasoline engine to be run on such lean mixtures, in excess of20:1 air-fuel ratio, that the levels of carbon monoxide and oxides ofnitrogen are simultaneously very low.

We claim as our invention:

1. A liquid fuel vaporizing device for a spark ignition enginecomprising:

at least one heat pipe, said heat pipe being in the form of a sealedvessel;

a narrow-range boiling point heat transfer fluid, said fluid beingdisposed in said sealed vessel;

said heat pipe being disposed so that the liquid phase of the fluidcollects in a heat-receiving zone, the vapor phase of the fluid beingarranged to condense in a heat transfer zone where heat is required tovaporize liquid fuel upstream of the point where the fuel is mixed withthe main stream of combustion air for said engine;

the heat-receiving portion of said heat pipebeing disposed to extendinto the exhaust manifold of the engine with the outer surface of saidsealed vessel in contact with the exhaust gas of the engine; and

a fuel conduit disposed in heat transfer relationship with the heattransfer zone of the heat pipe.

2. A device as claimed in claim 1, in which the heat pipe is providedwith additional cooling means which are arranged to remove heat from thevapor phase of the heat transfer fluid,

3. A device as claimed in claim 1 in which the inner surface of the heatpipe is lined with a wick-like structure for transporting the heattransfer fluid when in the liquid phase of capillary action.

4. The device of claim 1 in which the heat pipe contains a quantity ofgas which is non-condensable within the intended range of operation ofthe heat pipe.

5. The device of claim 1 in which the heat pipe is formed by a pluralityof individual pipes.

6. The device of claim 1 and in addition a secondary air ductcommunicating with said fuel conduit so that the fuel can be vaporizedin the presence of a secondary stream of air prior to said fuel mixingwith said mainstream of combustion air.

7. The device of claim 6, and in addition a main air duct for supplyingthe mainstream of combustion air, said main air duct converging until itcombines with said secondary air duct whereafter said combined main andsecondary air ducts diverge.

8. The device of claim 1 wherein the liquid phase present in the heatpipe occupies less than 40 percent of the total volume of the last pipe.

9. The device of claim 1 in which the heat transfer fluid has a boilingpoint of between C and 300C.

10. The device of claim 9 in which the heat transfer fluid is 2-octanol.

11. The device of claim 9 in which the heat transfer fluid is decane.

12. The device of claim 9 in which the heat transfer fluid is tetralin.

13. The device of claim 1 and in addition a return passageway formed insaid heat pipe for returning condensate form said heat transfer zone tosaid liquid zone.

14. A method for vaporizing the liquid fuel supplied to a spark-ignitioninternal combustion engine comprismg:

boiling an intermediate fluid enclosed in a sealed container in a hightemperature region;

using the latent heat of condensation of the vapor of said fluid tovaporize a fuel charge for the engine prior to its admixture with themainstream of combustion air, and

admixing said vaporized fuel and the mainstream of combustion air.

t t 10K gage UNITED STATES PATENT OFFICE v CERTIFICATE. OF CORRI!)(: 'I;IO1 I Patent No. 63,338 Dated October 9, 1973 I Roger Liridsay, AlunThomas, Ian G. Finley and John L. Wilson It is certified thaterrorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

"In column 2, line 36, "Figure 7" should read. Figure 8 In column 2,line 44, "Figure 11" should read Figure l2 Signed and sealed this 5thday of November 1974.

(SEAL) Attest:

McCOY GIBSPN JR. c. MARSHALL DANN Attestlng Officer Commissioner ofPatents

1. A liquid fuel vaporizing device for a spark ignition engine comprising: at least one heat pipe, said heat pipe being in the form of a sealed vessel; a narrow-range boiling point heat transfer fluid, said fluid being disposed in said sealed vessel; said heat pipe being disposed so that the liquid phase of the fluid collects in a heat-receiving zone, the vapor phase of the fluid being arranged to condense in a heat transfer zone where heat is required to vaporize liquid fuel upstream of the point where the fuel is mixed with the main stream of combustion air for sAid engine; the heat-receiving portion of said heat pipe being disposed to extend into the exhaust manifold of the engine with the outer surface of said sealed vessel in contact with the exhaust gas of the engine; and a fuel conduit disposed in heat transfer relationship with the heat transfer zone of the heat pipe.
 2. A device as claimed in claim 1, in which the heat pipe is provided with additional cooling means which are arranged to remove heat from the vapor phase of the heat transfer fluid.
 3. A device as claimed in claim 1 in which the inner surface of the heat pipe is lined with a wick-like structure for transporting the heat transfer fluid when in the liquid phase of capillary action.
 4. The device of claim 1 in which the heat pipe contains a quantity of gas which is non-condensable within the intended range of operation of the heat pipe.
 5. The device of claim 1 in which the heat pipe is formed by a plurality of individual pipes.
 6. The device of claim 1 and in addition a secondary air duct communicating with said fuel conduit so that the fuel can be vaporized in the presence of a secondary stream of air prior to said fuel mixing with said mainstream of combustion air.
 7. The device of claim 6, and in addition a main air duct for supplying the mainstream of combustion air, said main air duct converging until it combines with said secondary air duct whereafter said combined main and secondary air ducts diverge.
 8. The device of claim 1 wherein the liquid phase present in the heat pipe occupies less than 40 percent of the total volume of the last pipe.
 9. The device of claim 1 in which the heat transfer fluid has a boiling point of between 100*C and 300*C.
 10. The device of claim 9 in which the heat transfer fluid is 2-octanol.
 11. The device of claim 9 in which the heat transfer fluid is decane.
 12. The device of claim 9 in which the heat transfer fluid is tetralin.
 13. The device of claim 1 and in addition a return passageway formed in said heat pipe for returning condensate form said heat transfer zone to said liquid zone.
 14. A method for vaporizing the liquid fuel supplied to a spark-ignition internal combustion engine comprising: boiling an intermediate fluid enclosed in a sealed container in a high temperature region; using the latent heat of condensation of the vapor of said fluid to vaporize a fuel charge for the engine prior to its admixture with the mainstream of combustion air, and admixing said vaporized fuel and the mainstream of combustion air. 